Various techniques are applied in the mapping of subsurface formations for hydrocarbon exploration. Seismic techniques extract information about subsurface structures from anelastic (i.e., approximately elastic) energy reflected and/or refracted by the subsurface structures. Electromagnetic techniques measure electromagnetic fields induced in subsurface structures and extract information about the structures from the fields. Included in the information (in both techniques) is information concerning the fluid content of the pore space within the formations.